OK so I opened up the watch. I noticed that the back plate sprung back, mostly at the side of the crack:
Looking at the inside of the back plate, there seems to be a blob of some translucent material, with some excess around one of the pogo pins:
Could some of that translucent material have gotten caught between the back plate and the pogo pins' contacts, causing outward pressure on the back plate leading to the crack? On the one hand that seems consistent with the position and shape of the crack; on the other hand, it doesn't take excessive force to push the back plate into contact with the body.
Here's a picture of the rest of the internals with the battery, which doesn't look swollen to my (inexperienced) eyes:
I guess without a swollen battery we still have a bit of a mystery on our hands, but I'd definitely appreciate receiving a new watch body. I just hope that another part of the watch isn't the cause of this.